How do I show the pcp how far the dementia has progressed in such a short visit?

I video taped it and when that didnt work, forwarded all calls between 9 pm and 8 am to his cell phone which he accidentially called me from.

This behavior is so typical in humans, sane and those with dementia. First, get a Power of Attorney and alert the doctor that the doctor MUST talk with you and LISTEN. Document everything in detail, times, places, dates, details. Once you have plenty of examples, write a formal letter to the doctor stating all of the facts and your concerns. They should have this before the appointments and it should be visible in her chart. You can also ask the local Office on Aging how best to deal with these situations - that is what they are there for - to help seniors. Another important choice is that perhaps you are NOT dealing with the appropriate doctor. Sometimes you have to experiment before you find the right one. She is not the only senior doing this. Sometimes doctors are in a rush and don't want to be bothered or listen. Do you have another family member who could be a witness and attest to this? Good luck. And the meantime, get VERY TOUGH with your mother and lay down the law and boundaries and let her know that there will be severe consequences if she does not cooperate fully.

Is there anyway you can make an appt for yourself and just take your mother along, make sure you have plenty of time with the doctor to discuss several things (about you) and ask your mother to join in the conversation and hopefully he might see the decline.  Can they have your mother do a test (for dementia) while she is at her appt.  they can ask her something to repeat and they should wait a couple minutes and then ask her to repeat the answers or whatever, sometimes they make them draw a clock or something else to see if they can remember.  IF the doctor leaves the room, ask them to remain outside the door and listen to her conversation (it might seem sneaky) but they might be able to pick up on her mental status.  Other than tape recording (which could be considered wrong without the persons consent) how they act or whatever, maybe someone else has a better suggestion.  wishing you luck.

I would find another Doctor for your Mom asap. There are Doctors who are Geriactric Specialists. A good physician will give her cognitive 'tests' while she is in the office which will confirm her memory failure and other issues. If she has episodes, record them on your cellphone and show the Dr. There are several stages of dementia and it is important that her Doctor determine what level she is in and to follow and document her progress. My Mom would also get in front of the Doctor and smile and tell her everything is great.....until the memory tests are given, those cannot be 'faked'! My Mom has had dementia for the past few years and is declining rapidly. My husband and I care her caregivers and we have found that her behaviors (OCD, etc.) comes in cycles. This week it might be playing with her spinner incessantly, last month it was non-stop pushing all the buttons on her remote until the batteries run out. I have found that distracting the behavior works best, as much of the 'tic' behavior comes from anxiety, My Mom will do adult coloring books for hours, used to do jigsaw puzzles and loves to read. She has little recall over what she just ate five minutes ago but can talk for hours about the 1950's and remembers details that are astounding. Dementia is a progressive, debilitating and heartbreaking disease. A good physician will partner with you in your Mom's long-term care. If her current physician isn't supporting you and giving you everything you need to care for your Mom, it's time to find a new one. A geriatric specialist would be ideal. You can also inquire about medication to help ease the anxiety that is likely driving the some of the behaviors. A good physician will support and guide you through this process and will give you the resources you need to get through this without losing your sanity.

That happens OFTEN. What I did, was write-up a single letter paper description, in clear, concise sentences [just the facts]. Dated it, labeled it with her name and took it to the next Doc Appointment...and turned that in when I signed in, and said [kinda loudly so others can hear, but still reasonable volume]
"please make sure the Doctor reads this, and, enters it into her chart; it is important information I was Unable to tell them in the very short office visits allotted."
That really helped stop a problem before it got started.
Your mileage may vary, depending on how clearly you describe her changes, and whether to Doc actually reads it before seeing your Mom for appointment.
The Doc my mom saw, NEARLY was conned by mom's teen-like flirting while asking for serious pain drugs.
But instead, he at least had scanned over it before the appointment--so when Mom started laying it on thick, the Doc excused himself to "check something in the other room"--and I watched him take more time to really read it...when he returned to the exam room, he explained he'd write a prescription for her for a "very powerful pain med". Mom bought it hook, line, and sinker. She was fairly giddy for having pulled her con. But...that was blown to bits once in the car, when she asked if I knew that drug? I read the RX, which was some other name for basically, a strong version of something like ibuprofen. When Mom demanded answer, it was best to tell her...when she learned the Doc had busted her con, she went from sweet, flirty, to instant Rage. Threw down the wadded-up RX, carrying on lividly, bad-mouthing drugs, docs these days, and sliding into everything else in life that had been bad, becoming a veritable Niagara of horrible temper, pain, and sadness.
She later retrieved the wadded-up RX and hid it in her wallet. She still had it a few years later, "just in case" [despite her being a former nurse, she never questioned that it would be outdated by then, so not usable].
Mom was a supreme "showtimer"; skilled at using about every trick in the book to survive; she retained that skill long after need of it. She sat with social workers fro your County for TWO hours....but every approx. half hour, she took a powder to her room for about 10 to 15 minutes, returning to the assessment ready to handle another segment of showtiming. Rinse/repeat that for 2 hours. The social workers NEVER saw through that; they only saw her deliberate construct that appeared a level of normal. Yet I knew, and had a hard time getting the doc. workers to hear me. So none of them wold document anything I told them verbally.
But that Doc who went aside to read my letter to them for her chart...he listened, and acted right; if he had not read that letter, he would have never understood that Mom was off-and-on suicidal, mentally and emotionally harmed and hurting her whole life, and had misused prescription drugs, and more...and he would have missed that to the potential endangerment of Mom, since her MO has been to collect pills and take them with alcohol as much as she can get ahold of.
I know that patient records are mostly digital now.
I think they still all have to keep a paper chart, of some kind, and scan it to digital. But something in writing that is formally entered into her chart, can also be digitized into her chart. And in-writing is more meaningful legally.
And it gets noticed--especially when you make sure you are heard when asking them to put it into her chart upon signing in, and repeated to the one who guides you back to see the Doc on the way into the exam room.
And keep a copy of it, labeled with the time/date you requested it be put in her chart. By doing that, it is due-diligence for someone doing elder care, even if from a distance.